The present invention relates to a method and apparatus for the orientation of plastic pipe and, more particularly, for the orientation of plastic pressure pipes used in oil transportation, natural gas supply, portable water systems, irrigation systems, and chemical plant installations.
The orientation of plastic pipe results in an enhancement in strength of the articles in a given direction by orienting the molecules in the plastic material in that direction. In the case of plastic pipes, orientation is effected in the hoop direction, that is, hoop orientation, whereby the pressure resistance of the plastic pipe increases and/or in the longitudinal direction of the plastic pipe, that is, axial orientation, whereby the tensile strength of the pipe increases.
Methods and apparatus for orienting plastic pipes are known in the prior art. A typical method and apparatus is disclosed in European Patent Application 0 371 769. The method and apparatus disclosed in the '769 patent obtains axially oriented plastic products by drawing a tubular plastic material over a mandrel. The advantage of the method and apparatus of the '769 document is that the tubular material is drawn over an expanding mandrel without the need of liquid lubricants. Related methods and apparatus are discussed in a number of publications of the inventors of the '769 document including the following:
(1) PLASTICS AND RUBBER PROCESSING AND APPLICATIONS, Vol. 10, No. 2, 1988, pages 85-91, "The Preparation and Properties of Die-drawn Polyvinylchloride Tubes" by A. Selwood et al. PA1 (2) PLASTICS AND RUBBER PROCESSING AND APPLICATIONS, Vol. 25, No. 6, 1996, pages 287-290, "Biaxially Oriented Polymer Tubes by Die Drawing" by A. K. Taraiya et al.
In the methods and systems described above the plastic tubing is heated in an empty chamber by means of electrical heaters. The plastic tubing must be rotated in order to obtain some degree of uniform heating. Because of the foregoing, the amount of time necessary to get the plastic tubing to the required temperature is significant. In addition, the method and apparatus relies on cooling of the mandrel by forced convection air. This results in an inefficient cooling of the plastic pipe after expansion. Inefficient cooling results in unwanted relaxation of the plastic pipe which diminishes the mechanical properties of the final product.
Accordingly, it would be highly desirable to provide a method and apparatus for the orientation of plastic pipe which overcomes the deficiencies noted above.
Accordingly, it is a principal object of the present invention to provide a method and apparatus for the orientation of plastic pipe which results in an efficient uniform heating of the plastic pipe above the glass transition temperature.
It is a further object of the present invention to provide a method and apparatus for the orientation of plastic pipe which provides for a quenching of the oriented plastic pipe so as to eliminate the problem of relaxation associated with the prior art.
It is an additional object of the present invention to provide a method and apparatus which allows for the internal calibration of the orientated plastic pipe so as to prohibit collapse of the oriented plastic pipe as it is drawn off the orienting apparatus by a take-off unit.
Further objects and advantages of the present invention will become apparent hereinbelow.